'We'd like to adapt some of our existing software for a very specific task, can you help?' declared TWI's client, a major materials manufacturer. 'Using MicroSoft Excel could you generate a spreadsheet for the estimation of accumulated plastic strain during coiling and uncoiling of the component we manufacture.'
TWI rose to the challenge. The client concerned makes longitudinal stock for subsea umbilical applications and performs coiling and uncoiling operations as an unavoidable part of the manufacturing process. The component is plastically deformed during these operations and imposes a varying number of strain cycles.
The maker recognises that some customers call for an estimate of the accumulated plastic strain (APS) and further appreciates that the standard industry practice, used to make such estimates, is often significantly conservative.
It has already developed a spreadsheet to calculate APS but wanted to refine it. TWI was asked to review the existing spreadsheet and modify it to include the effects of strain hardening and the orientation of the component.
The new spreadsheet is a significant improvement. It does not assume the material is perfectly plastic and, more significantly, it does not assume a constant plane of bending. A kinematic hardening law from Lemaitre and Chaboche suitable for 'round house' stress strain curves has been used. The spreadsheet analyses four locations around the circumference of the pipe.
It has two operating modes. Strain can be accumulated on two planes with four reporting positions. Alternatively, a single result can be calculated by assuming that the plane of bending is constant. The single result mode is overly conservative.
The revised spreadsheet contains eleven rows of manufacturing operations having a specified event sequence. Other sequences can also be accommodated. The operations are specified by a code. And other cells can be used to define the bending plane and the diameter of bending if appropriate.
The revised spreadsheet is a significant improvement on the methods that are generally used today because the full stress versus strain curve is used and the plane of bending is used in the assessment. Previous spreadsheets have assumed erroneously that the material does not harden.
TWI's work concluded by recommending that calculations of accumulated plastic strain should always include 90 degree changes to the bending plane of the component during manufacture.
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